1. Field of Invention
The present invention relates to a measuring method, according to the preamble of claim 1, for measuring the particle size and shape distribution of powder material. By means of the method, a surface is formed in a controlled manner of the powder material, which can be imaged in precisely controlled exposure conditions.
2. Description of Related Art
FI patent 20000493 discloses an analysis method for a moving granular material. The method is not suitable for accurate optical grain-size distribution analysis, precisely on account of the movement of the material being studied.
U.S. Pat. No. 5,239,358 discloses a method for removing foreign particles from a powder sample. The central component of the invention is a rotating sample plate. Precisely on account of the rotating plate, the method is not suitable for accurate optical grain-size distribution analysis.
JP publication 10010033 discloses a measuring method for a moving powder substance. In this publication, the light passing through a sample is studied. The measuring method does not appear to be accurate.
U.S. Pat. No. 6,122,042 discloses several methods for optical analysis. In the method, comparison image pairs of a stationary sample are not created.
WO publication 03062804 discloses a method for optical analysis. In the method, the fine-particle distribution is not determined, nor are comparison image pairs created from a stationary sample.
It is also known that the particle-size distribution can be relatively reliably defined from the surface of a powder or granular material (scientific publication: Laitinen, N., Antikainen, O., and Yliruusi, J. (2002) Does a powder surface contain all necessary information for particle size distribution analysis? European Journal of Pharmaceutical Sciences, 17(4-5), 217-227). In this case, the subject of the measurement of particle size is an undispersed powder, from which a level surface is made. Thus the method cannot be used to measure a property of a single particle, but of a larger group of particles.
The determining of a particle distribution from a powder surface, using the prior art, takes place according to the following description. For example, the most level surface possible is made from a powder sample in a sample container. In the method, a light source is used to illuminate the sample surface, in such a way that shadows are formed on the surface. The shadow formation is utilized by converting the surface information of the powder to depict the particle size of the powder material. To illuminate the sample it is possible to use, for example, two light sources placed symmetrically on either side of the sample. The sample is then illuminated by alternating the light sources and taking a digital image using a CCD camera, in such a way as to obtain two digital images. The sample must remain stationary, as first of all the difference is calculated between the two grey-tone matrices from which the digital image is formed. For a theoretically completely even sample, the difference matrix is formed of elementary units that are zero. For a real difference matrix, the elementary units receive values that are integers between −255-+255 (if a grey-tone range of 0-255 is used). The next stage of the calculation determines how many elementary units of the difference matrix have received a certain value of the 511 possible values.
The known measuring method is based on the fact that a sample with a certain grain-size distribution will always form a grey-tone difference distribution that is specific to it. To be able to use the measuring method, a model must be made for the pitch of the actual size distribution of the grain of the grey-tone difference distribution, which will convert the measurement result obtained into a size distribution. In the known method, a PLS (partial least squares) model is used, which is based on principal-component analysis. The independent variables are the 511 values of the distribution of the elementary units of the aforementioned grey-scale matrix while the response variable is the percentage mass fraction on each screen of the screen series of the screen analysis of the grain batch. With the aid of the model in question, it is possible to determine the grain-size distribution, which corresponds to the result obtained by screen analysis of the same powder material. A corresponding model can be similarly calculated for other generally used methods for determining particle size.
In the known method, stable collimated (parallel) and direct current light sources are used. The illumination can be arranged using two stable light sources, or using a single light source, the light provided by which is dispersed optically, in such a way as to make it possible to illuminate the sample symmetrically in two directions. A matrix-type (CAD) camera or similar, or generally any opto-electric camera or image-recording method whatever can be used to detect the image signal. Imaging can also be performed using only a single light source, in which case that the sample is rotated. Afterwards, the images can be combined according to the above matrix-difference principle. In the manners described above, it is also possible to take several images of a single sample surface by rotating the sample, or by altering the imaging angle. In this way it is possible to obtain more information from the surface of the powder.
The material being studied can be a surface created in a suitable manner from powder, granular, or other multi-component samples. The surface can be created by wiping with a suitably flat spatula or plate. Repeatability is a problem in the sample preparation method of the aforementioned measurement technique. In the method in question, the powder surface is made by pouring an excess amount of powder into a sample container, after which the excess is removed using a suitable plate, while simultaneously attempting to create a level surface from the powder. The surface is leveled by pulling the levelling plate horizontally over the upper edge of the sample container. When the surface is leveled in this manner, unevenness can form on the surface due to the manufacture of the sample, for example, large particles in the powder can cause wipe traces on the surface. It can also be difficult to level powders with small particles that cohere easily to each other (cohesive powders), when using the levelling method in question. The aforementioned problems will lead to errors in measurement.